Most radio frequency receivers convert received radio frequency (RF) signals to lower frequencies (e.g., baseband or intermediate frequencies) employing mixers. The filtering and amplification performed in processing the RF signal the lower frequencies is less expensive and more accurate than processing at the RF frequency. Typically, a modulated RF signal is combined in a mixer with a local oscillator (LO) signal to produce an intermediate frequency (IF) signal which can then be amplified and detected to recover the information modulated onto the RF carrier. The process can also be reversed by mixing a LO frequency signal with an IF signal to produce a modulated RF carrier that can be amplified and transmitted as a modulated signal.
A mixer is a nonlinear device containing either diodes or transistors, the function of which is to combine signals of two different frequencies in such a way as to produce energy at other frequencies. Various mixer parameters, such as bandwidth and inter-port isolation, must be optimized to produce devices capable of performing in modern RF systems. In recent years there has been an increase in sub-millimeter-wave receiver applications requiring easily producible and improved performance mixer technology at extremely high frequencies. Mixing an input RF signal with a LO signal yields frequency products below and above the RF and LO frequencies. Each frequency product corresponds to the sum of the input RF and LO frequencies, while the lower frequency product corresponds to the difference between the input RF and LO frequencies.
Some mixer types include single-ended, single-balanced, double-balanced, and double double-balanced (also called triple-balanced). All of these mixer types are three-port devices and comprise an input port (the RF port), a local oscillator input port (the LO port), and an output port (the IF port). Single-ended mixers are the simplest type and are realized using only a single diode. The LO, RF and IF ports are separated only by filters to provide some degree of inter-port isolation. Single-ended mixers, however, have a narrow bandwidth, limited dynamic range and poor inter-port isolation. Broader bandwidths and better isolation can be obtained with a single-balanced mixer. A single-balanced mixer consists of two single ended mixers. The mixer diodes are fed by the LO and RF signals. Harmonic mixers have been utilized in which the principal output is at a particular harmonic of the LO frequency. A more recent type of even sub-harmonic pumped mixer uses two diodes connected in parallel and opposing polarity, referred to as “antiparallel diodes”. The sub-harmonic frequencies provide output frequencies as combination of only even harmonics of the local oscillator frequency.
Prior art mixers have a number of disadvantages well known in the art. Among these disadvantages are port-to-port isolation, limited bandwidth, particularly intermediate frequency bandwidth, relative complexity and difficulty of implementation in compact form suitable for incorporation in monolithic microwave integrated circuits (MMIC's). MMIC's are typically constructed using elemental semiconductor integrated circuit (IC) wafer processing technology on and/or in such wafers. Typically, broadband mixers are made with lumped elements or other structures that are compatible with IC fabrication techniques and geometries. In particular, it is important that they be of comparatively small size so as to not occupy disproportionately large substrate areas compared to the semiconductor diodes, transistors, etc., which mix the signals, or compared to the amplifiers or other signal processing elements that may be included in the MMIC. Such concerns are important in the frequency range above one GigaHertz (GHZ) where the sizes of distributed circuit elements are unwieldy. In particular, for broad-band applications, many lumped or distributed elements are employed which, in monolithic applications, require substantial die area. Therefore, conventional microwave mixers have limited bandwidth ranges resulting in customized mixers for radios that are designed to operate in specific bandwidths. A radio manufacturer of radios of varying bandwidths will then need to store multiple mixers of varying bandwidth, so as to meet the specific bandwidth requirements of the different radios.
Another problem with mixers is that the introduction of direct conversion receivers with their strong on-board LO signals present the problem of interference. More specifically, spurious LO leakage from the receiver into the antenna can cause in-band interference with other nearby receivers tuned to the same channel. Therefore, such a receiver would require a very high level of isolation between the local oscillator and the antenna in order to avoid swamping or saturating receivers of any nearby radios. Such high levels of isolation are very difficult to achieve due to stray capacitances directly coupling local oscillator energy into the antenna. Known methods addressing the problem of LO leakage include providing isolation in the RF path to the antenna, inserting an isolator such as filters on the mixer and, reducing the local oscillator drive power to very low levels. However, these methods have shortcomings, since they have been found to be detrimental to dynamic range and degrade sensitivity, as well as being expensive to implement and increasing die space.
Therefore, there continues to be a need for improved broadband mixers and methods of mixing signals that use fewer components, are easy to construct and/or employ elements that are readily integratable in and/or on MMIC's.